There is a growing need for the creation of energy efficient processes for the separation of multicomponent mixtures. The need for energy efficiency in this context has been augmented recently by environmental concerns. The separation of multicomponent mixtures is desirable for many reasons, most notably for use and production of a variety of industrial products. The distillation of propane and propylene illustrates this problem. This process is well known to be a very energy-intensive process and poses serious environmental risk.
Recent advances in material sciences have brought promise to this area, as a plethora of new materials have become available that can be used to selectively adsorb certain molecules from a gas. Modern adsorption processes have taken advantage of these materials as many of these processes employ the use of two different adsorption materials to separate multicomponent mixtures in the air.
Pressure swing adsorption (PSA) relies on swinging or cycling pressure over a bed of adsorbent through a range of values. In PSA processes, a gaseous mixture is conducted under pressure for a period of time over a first bed of a solid sorbent that is selective, or relatively selective, for one or more components, usually regarded as a contaminant, to be removed from the gaseous mixture. For example, a feed can be introduced into a PSA apparatus at a feed pressure. At the feed pressure, one or more of the components (gases) in the feed can be selectively (or relatively selectively) adsorbed, while one or more other components (gases) can pass through with lower or minimal adsorption. A component (gas) that is selectively adsorbed can be referred to as a “heavy” component of a feed, while a gas that is not selectively adsorbed can be referred to as a “light” component of a feed. For convenience, a reference to the “heavy” component of the feed can refer to all components (gases) that are selectively adsorbed, unless otherwise specified. Similarly, a reference to the “light” component can refer to all components (gases) that are not selectively adsorbed, unless otherwise specified. After a period of time, the feed flow into the PSA apparatus can be stopped. The feed flow can be stopped based on a predetermined schedule, based on detection of breakthrough of one or more heavy components, based on adsorption of the heavy component(s) corresponding to at least a threshold percentage of the total capacity of the adsorbent, or based on any other convenient criteria. The pressure in the reactor can then be reduced to a desorption pressure that can allow the selectively adsorbed component(s) (gas(es)) to be released from the adsorbent. Optionally, one or more purge gases can be used prior to, during, and/or after the reduction in pressure to facilitate release of the selectively adsorbed component(s) (gas(es)). Depending on its nature, a full PSA cycle can optionally be performed at a roughly constant temperature. As PSA is usually enabled by at least adsorption and usually occurs on gaseous components, the terms “adsorption”/“adsorbent” and “gas(es)” are used as descriptors in the instant specification and claims, without intending to be limiting in scope, even though “absorption”/“absorbent”/“sorbent”/“sorption” and “component(s)” may be more generally applicable.
Multiple beds can be used to enable a complete cycle, where typically every bed sequentially goes through the same cycle. When a first PSA reactor satisfies a condition, such as the adsorbent in the reactor becoming sufficiently saturated, the feed flow can be switched to a second reactor. The first PSA reactor can then be regenerated by having the adsorbed gases released. To allow for a continuous feed flow, a sufficient number of PSA reactors and/or adsorbent beds can be used so that the first PSA reactor is finished regenerating prior to at least one other PSA reactor satisfying the condition for switching reactors.
U.S. Pat. No. 4,744,803 discusses the use of a four-step PSA cycle. The adsorption bed is first pressurized by entry of gas from the bottom of the bed while the top end of the bed is closed. This is referred to as the pressurization step. The next step is high pressure feed, wherein feed gas enters under pressure from the top of the column and effluent is allowed to escape from the bottom of the column. At the conclusion of this step the column is closed at both ends and the pressurized gas is then released by opening the top end of the column. This is referred to as the blow down step. After the pressure has been reduced to a predetermined level by blowdown, the column is next purged of remaining product by feeding recycled product gas into the bottom end of the bed and allowing the gas remaining in the column to be forced out of the top end as effluent. This step would normally be terminated at the point where the purging gas reaches the top end of the column. The effluents from the blowdown and purge steps contain the component adsorbed by the column. This is generally referred to as the secondary product of the column. The primary product is the component or component which pass through the bed unadsorbed, i.e., the high pressure feed effluent.
U.S. Pat. No. 4,744,803 also provides a PSA system where adsorbents 1 and 2 are disposed in 4 beds, which are respectively selective for species A and B. This system uses the secondary product of one bed (the blow down and purge effluent) partially or wholly as the feed for another bed containing a different, or complementary adsorbent. This system, however, has the disadvantage that compressors are required to feed the secondary product of one feed to another. Thus greatly increasing the energy required to operate the system.
Another process known in the art is dual reflux PSA, described by Diagne et al., Ind. Eng. Chem. Res., Vol. 34 No. 9, 1995. In this process, an intermediate feed position divides an adsorber column into rectifying and stripping sections, and this process relies on external compression to create a constant product.